And so the iOS-ification of Mac OS X continues. Apple has just announced that all applications submitted to the Mac App Store have to use sandboxing by March 2012. While this has obvious security advantages, the concerns are numerous - especially since Apple's current sandboxing implementation and associated rules makes a whole lot of applications impossible.

First, I agree with you that it's difficult for a legacy operating system to make application sandboxing mandatory. Some compatibility will be broken, sooner or later. However, OS vendor control on applications is not the only way to do that. You can also do it the Microsoft way, by pushing an OS release that breaks compatibility, but is advantageous in other ways, like Windows x64 breaks DOS compatibility. At some point, everyone will use the new release, although it can take some time.